Recent evidence has suggested that a prolactin-like substance of hypothalamic origin plays an important role in female mating behavior in rats by stimulating lordosis, a simple motor reflex. This observation explains the repertoire of known reproductive functions involving prolactin. It also suggests that prolactin acts as a neuromodulator as well as a hormone. Studies will therefore directly assess the capacity for localized synthesis of authentic prolactin in the rat brain. Production of this protein requires synthesis of the messenger RNA that codes for it; the use of a cloned complementary DNA probe in RNA/DNA hybridization studies makes it feasible to identify and structurally characterize minute quantities of the mRNA, as well as to localize the cells which contain it. Characterization of the newly synthesized protein itself will be made, using tissue obtained from rats following direct infusion of radiolabeled amino acids onto the hypothalamus. protein extracts will be immunoprecipitated with a prolactin antibody; the immunoprecipitates will be compared to pituitary prolactin following gel electrophoretic fractionation. Lordosis is an estrogen/progesterone-regulated behavior. The proposed role for hypothalamic prolactin suggests that its production is also under sex steroid regulation. This possibility will be directly tested by mRNA quantitation studies on brain tissue from animals in different endocrine states. The potential for lordotic behavior is programmed by sex steroid action on the brain during early postnatal development and can be experimentally manipulated. Therefore, experiments will determine whether altering the hormonal state of neonates permanently changes the prolactin-synthesizing system in the adult brain, either in cell number or in potential for steroid regulation. The onset of prolactin synthesis in the pituitary lags behind that of other pituitary peptide hormones. Studies on developing brain will determine whether onset of prolactin gene expression is a relatively late event in this tissue as well. Finally, studies will determine whether individual cells in the developing pituitary or brain have the capacity to simultaneously express more than one peptide hormone gene, and whether this capacity is lost as differentiation of these organs proceeds.